Propagation is the obvious answer to this question. What is propagation then? When your website address is entered into a web browser, the computer requests the IP address of the server where your site is from; that is, your local Internet Server Providers (ISP) DNS records. So, if the website is not listed in the records, it queries the registrars to determine if the DNS Start Of Authority (SOA for short) is for your site. For instance, if you are using your registrar's name server as your SOA, it looks up the "A" record for your domain and returns the IP address of the server. On the other hand, ıf you are using our name servers, the registrar points the browser to our DNS servers to determine the IP address for your domain name. Afterward, the request is sent to the server where the domain is hosted and then provides the browser with your site.
In order to speed the loading of websites, each ISP caches a copy of the DNS records for a specific time, in other words, they make copies of the registrar master DNS records and read from them locally in lieu of making a direct request to the domain registrar every time a request for your site is made. This enhances or speeds up web surfing by decreasing the return time it takes for a browser to request a domain lookup and get a response. Also, it reduces the amount of traffic on the web.
Keeping that in mind, it is advisable to allow up to 24 hours for your domain to propagate fully on your end since it is an automated process that we ourselves can't speed it for you. Notwithstanding, once your domain propagates fully, you will be able to access your site without any further delays.